1. Field of the Invention
The present invention relates to a speed-and-phase control circuit adapted to control the rotational speed-and-phase of a rotary machine.
2. Description of the Prior Art
A speed control circuit is frequently employed for constant speed control of a rotary machine, such as an electric motor. Such a conventional speed control circuit is responsive to an alternating current signal that is generated by the rotary machine and has a frequency which is proportional to the number of revolutions of the rotary machine per unit time. The alternating current signal is converted by the constant speed control into a direct current signal whose amplitude is proportional to the frequency of the alternating current signal. The direct current signal is then compared to a reference voltage signal so as to produce a difference signal. The difference signal is used as a feed-back signal and, thus, allows the constant speed control circuit to provide the constant speed control of the rotary machine.
If synchronization of the revolution number of the rotary machine with a reference signal of predetermined frequency or if coincidence of rotation phase with the phase of a signal of predetermined frequency is required, however, a rotation phase control circuit is needed in addition the speed control circuit described above. A device having both a phase control circuit and a speed control circuit is electronically complicated and is difficult to build and maintain.
Several attempts have been made to simplify the device having both a phase control circuit and a speed control circuit. One such approach employs a circuit which exhibits several major deficiencies. For example, the circuit employs a monostable multivibrator whose time constant inherently varies with temperature, and, thus, the circuit characteristics are prone to be unstable. Furthermore, the circuit needs capacitors to form the monostable multivibrator, and, therefore, in the case where the circuit is manufactured in the form of an integrated circuit, it is necessary to provide the capacitors externally.